A binary material such as GaN conventionally appears in a polar form, with a particular orientation of the crystalline unit cells of the material. This orientation is expressed at both surfaces of the substrate by asymmetry: one of the two faces will be the “Ga” face, whereas the other opposite face is the “N” face. It is known that growing an active layer of electronic components on polar GaN material oriented along the c axis is easier to achieve with good quality by starting with a Ga face rather than from a N face. Therefore, this involves certain constraints for achieving composite substrates comprising a transferred layer of polar GaN.
There essentially are two types of transfer methods with which such substrates may be obtained. A first known type of method consists of picking up a thin layer of GaN, from the rear “N” face of a GaN donor substrate, in order to transfer this layer onto a receiving substrate. Such a known method bonds the rear face of the GaN layer, as it is desired that the final obtained composite substrate has a front face, which corresponds to its “Ga” face. Regardless of the retained transfer technique, it is necessary to prepare the rear face of the GaN donor substrate before proceeding with the bonding with the receiving substrate. Indeed, an “N” face of a GaN layer requires treatments in order to notably reduce its roughness before bonding it with another surface.
This preparation therefore is a specific operation which corresponds to a cost.
A second known type of method does not require such a treatment of the rear face. In the second type of method, one proceeds with transferring a layer of material such as GaN, by bonding the front face (Ga face) of a donor substrate of this material onto an intermediate substrate. The front face of a GaN layer does not require any treatments as unwieldy as those which should be applied to a rear face of the same material for its bonding.
At the conclusion of this first bonding, a desired thickness of binary material such as GaN is retained on the intermediate substrate. An intermediate composite substrate is thereby obtained, the exposed face of which corresponds to the N rear face of the GaN layer. However, this “N” face is not the face that is desired on the front face of the composite substrate. At the conclusion of this first transfer, a second transfer of a layer of binary material such as GaN, is therefore achieved from the first receiver onto a receiving substrate. This second transfer may further be achieved hereby, e.g., thinning the intermediate substrate. A composite substrate is thereby obtained, the exposed face of which is of the “Ga” type without resorting to a constraining treatment of one rear N face of a GaN layer, in order to allow its bonding.
But this second type of known method requires achieving a transfer on an intermediate substrate, which burdens the manufacturing cost of the composite substrate.
It thus appears that the known methods for obtaining, through one or several transfers, a composite substrate including a layer of binary material such as GaN are associated with different drawbacks.